[Release SM 6.9] Cherry-Pick Fix GVN and SROA miscompilation of min precision vector element access#8370
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microsoft#8269) ## Summary Fixes three related optimizer bugs that cause miscompilation of min precision vector element access (`[]` operator) on `min16float`, `min16int`, and `min16uint` types at optimization levels O1+. Resolves microsoft#8268 ## Root Cause DXC's data layout pads min precision types (`i16:32`, `f16:32`). The HLSL change in `DataLayout::getTypeSizeInBits` makes vector sizes use alloc size per element (e.g., `<3 x i16>` = 96 bits), but scalar `getTypeSizeInBits(i16)` returns the primitive width (16 bits). This inconsistency causes three bugs: 1. **GVN ICE**: `CanCoerceMustAliasedValueToLoad` creates a padded-width integer (i96) then attempts a bitcast from the 48-bit LLVM vector type — assert fires. 2. **GVN incorrect store forwarding**: `processLoad` forwards a zeroinitializer store past partial element stores because MemoryDependence uses padded sizes for aliasing. 3. **SROA element misindexing** (primary bug): `getNaturalGEPRecursively` uses `getTypeSizeInBits(i16)/8 = 2` for element offsets, while GEP uses `getTypeAllocSize(i16) = 4`. Byte offset 4 (element 1) maps to index `4/2 = 2` instead of `4/4 = 1`, causing SROA to misplace or eliminate element stores. ## Changes **`lib/Transforms/Scalar/GVN.cpp`** - `CanCoerceMustAliasedValueToLoad`: Reject coercion when type sizes include padding (`getTypeSizeInBits != getPrimitiveSizeInBits`) - `processLoad` StoreInst handler: Skip store-to-load forwarding for padded types **`lib/Transforms/Scalar/SROA.cpp`** - `getNaturalGEPRecursively`: Use `getTypeAllocSizeInBits` for vector element size to match GEP offset stride - `isVectorPromotionViable`: Same fix for element size calculation - `AllocaSliceRewriter` constructor: Same fix for `ElementSize` ## Testing - All 6 min precision ArrayOperator tests (StaticAccess + DynamicAccess × 3 types) pass with the fix - Verified optimized DXIL output retains all 3 element stores with correct indices ## Co-authored This fix was investigated and implemented with the assistance of GitHub Copilot (AI pair programming). The root cause analysis — tracing the bug through `-print-after-all` pass dumps, identifying SROA as the culprit, and understanding the `getTypeSizeInBits` vs `getTypeAllocSize` mismatch — was a collaborative effort. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> --------- Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
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Cherry-pick PR (#8269)
Assisted by gh copilot.
Depends on: #8369
SHA ccba9f8